The current invention relates to the field of step ladders, and to the design of step ladders to permit a folding structure to be used as a self-supporting, or free-standing ladder, while maintaining an adequate degree of stability to prevent over-tipping and dangerous falls.
Step ladders provided a means for an individual to climb to a height for manipulating objects or performing work where the ladder is essentially free-standing; that is, the ladder depends solely upon its construction and erection to insure its stability, in comparison to standard extension ladders which are braced against a structure to be climbed.
A typical step ladder is designed to be folded into a convenient size for storage and carrying, and the requirement that it be portable is a further constraint, restricting the total weight of the ladder. Thus, a typical step ladder will be found to weigh thirty pounds or less weight, in order to insure that it may be easily handled, and it is typically designed to be folded into an essentially flat package of not much increased size over a fixed ladder of similar size.
The classic step ladder has a front ladder portion having two vertically ascending parallel members with angled steps periodically interposed between to permit climbing. At an upper end a rear leg section is pivotally attached, and a folding brace member is used to extend the ladder into an expanded, climbable configuration. The front and rear leg members and the cross-brace form the classic letter "A" in the extended position.
The stability of such a ladder is totally dependent upon the user's movement upon the ladder during use. The requirement that the ladder be portable tends to reduce the static weight of the ladder to as low a level as is consistent with minimum structural strength, and it will typically be found that such a step ladder will weigh under thirty pounds; when considering that a typical user will weigh one hundred and fifty pounds or more, this implies that in use practically all the weight involved in the dynamic couple of step ladder and user will be concentrated in the user and that there will be very little static weight tending to stabilize the ladder.
Thus, two forces degrade the stability of the ladder and create dangerous toppling propensities. The first is static stability; that is, the fact that the ladder will unconditionally fall if a line, extending downward from the combined center of gravity of the user and ladder, extends to a point outside the area demarked by the legs of the ladder. This is the classic condition of static instability and results inevitably in toppling. Because the ladder has such little weight relative to the user the actual location of the center of gravity of the user predetermines the stability. A typical individual's center of gravity is located approximately at the belt line, and prior art ladders have concentrated in maintaining stability by providing extensible retaining rails so as to by physical restriction or by suggestion and feel restrain the user from leaning excessively over the side of the ladder, producing an unstable condition.
A second, dynamic condition of instability exists because a user at the top of the ladder, in normal motion, is exerting a reactive force across a moment couple essentially equal to the distance from the foot of the ladder to the point of contact with the user; this can often be a six foot moment couple. Since this is a dynamic condition, induced by the motions of the user during work, the resisting couple is that solely of the weight of the ladder itself, which is considerably less.
The dynamic motion coupled from the user's motion imposes two separate forces to the ladder. The first is asymmetrical side thrusts. To the extent that the force of the user's weight is projected along the force of gravity but off centered from the center of gravity of the ladder a differential vertical loading is established downward along the leg. Since the total of the downward gravitational component along the leg must equal the total combined weight of the user with load and the static weight of the ladder, it should be apparent that significantly increasing the side thrust loading along one leg can result in a significant decrease in the thrust loading along an opposite leg. This can be observed in wooden step ladders of relatively filmsy construction on an uneven surface as an actual lifting of one leg off the ground; it is quite naturally an alarming condition to the user and occurs immediately before overturning and toppling. Furthermore, since ladders involve at least one lattice structure of vertical legs and steps, such uneven loading produces actual sideways stress upon the bottom portion of the parallel legs where they touch the ground. It has been observed by testing agencies that when ladders of typical construction are tested for this condition that some ladders will actually bend and fall at the forced detachment point of the leg and the lowest step due to the side thrusts generated.
The motion of the user also creates a torsional stress, twisting the ladder. To the inventor's knowledge, this form of stress has not been recognized or discussed in the literature; nonetheless, it is the inventor's belief that torsional stresses, in the form of twisting about the vertical axis of the ladder, is the most common cause of dynamic unloading of one or more ladder legs in response to the user's motion. The result is an alternative loading and unloading of the legs of the ladder which produces an effect called "walking" where the ladder moves or creeps across the floor as the individual shifts his weight. Such a condition is indicative of marginal stability in the ladder, and can produce extremely dangerous situations.
The recognition of the problems referred to above as static stability have led to the various forms of wide base ladders, most typically having an approximate triangular effect as in Harrison, U.S. Pat. No. 2,650,014. Harrison recognized the increased stability to be achieved by widening the footprint of the ladder by independently articulating the two rear support legs.
However, these triangular ladders have been unsuccessful principally because the effects induced by dynamic instability, especially torsional instability, have not been allowed for. The active independently articulating rear support legs have removed the inter-leg bracing typical in the older "A" frame style ladders. Of necessity, each of the individual rear support legs is a columnar structure or tubular structure of relatively restricted mass in order to provide a feasible ladder of suitably light weight. Each of these prior art structures has utilized some form of folding bracing to brace the legs individually against each other and against the front of the ladder. However, such folding bracing, whether that is shown in Harrison or in an earlier patent to Konigsberg, U.S. Pat. No. 1,778,898, has proven unsatisfactory in achieving adequate rigidity within the triangular ladder structure.
The current invention also recognizes the inherent safety hazard posed by the side braces on the older "A" frame ladders, and the lack of a positive locking mechanism for securing the legs of the ladder when open or closed.